Magnetically Focused Proton Irradiation of Small Volume Targets

Purpose/Objective(s)

Advances in imaging technologies are fueling a trend in radiation medicine necessitating the irradiation of smaller targets with greater conformity; however, as field size decreases, the peak to entrance dose performance and penumbra of proton beams is degraded by beam broadening due to multiple Coulomb scattering (MCS). Magnetic focusing of protons immediately before entrance into the patient could be used to counteract MCS leading to improved therapeutic ratios and decreased treatment times.

Materials/Methods

Magnets consisting of 24 segments of radiation hard samarium-cobalt adhered into hollow cylinders were designed and manufactured for preliminary preclinical testing. Two magnets were placed on our Gantry 1 treatment table, and proton beams with energies of 127 and 157 MeV, modulations of 0, 15, and 30 mm, and 8 mm initial diameters were delivered to a water tank using single-stage scattering. Depth dose distributions were measured using a PTW PR60020 diode detector and transverse profiles were measured with gafchromic EBT3 film. Monte Carlo simulations were also performed, both for comparison with experimental data and to further explore the potential of focusing in the context of proton radiosurgery.

Results

Preliminary Monte Carlo simulations of 5 mm diameter 100 MeV beams focused with 400 T/m magnet gradients produced beam spots at the Bragg peak that were comparable to unfocused 2 mm collimated beams. The focused beams displayed an ∼9x more efficient dose to target delivery, 21% larger peak to entrance ratios, 11% reduced Bragg depth major axis penumbras, 16% reduced Bragg depth minor axis penumbras, and 36% reduced entrance minor axis penumbras, compared to the 2 mm collimated beams. Preliminary experimental results showed 23–24% higher peak to entrance dose ratios and flatter spread out Bragg peaks for 8 mm modulated focused beams compared to unfocused beams.

Conclusion

The efficient production of small target beam spots with reduced entrance dose and penumbra has important clinical implications. Our results suggest that rare earth focusing magnet assemblies can reduce skin dose and beam number while delivering dose to millimeter-sized, nominally spherical radiosurgery targets over a much shorter time compared to unfocused beams. Immediate clinical applications include those associated with proton radiosurgery and functional radiosurgery of the brain and spine; however, expanded treatment sites can be envisaged as we gain further clinical experience with the system.